The color of e-liquid in the tank changes, while others don't

[Revealed]

Recently I have been using 3ml dct's for vaping and have noticed something odd. Some of the e-liquids will progressively change to a darker/brownish color while some don't. For instance, I have been using pineapple and mango both from ECblendflavors.com, 24mg 70PG/30VG in the dct's. I keep them both with me and have noticed that the pineapple one will darken over time, while the mango one doesn't. I also recently noticed this happening with green apple, 18mg 75PG/25VG from thevaporroom.net.
Is this normal and if it is, howcome this only happens on some?

 

[Vego]

I have noticed this on my Vision Extreme ce2 type carto tanks. It is my understanding (I got this from Steeljan) that some of the lighter, thinner elements that make up a juice will wick faster than the thicker ones. So it seems there is some separation going on in the juice. What happens is the thicker, heavier elements are left to concentrate in the tank, resulting in a darker looking juice. I don't know why it is happening with some and not others, though I imagine it all has to do with the different ingredients that go into the recipe. To be honest, I've noticed it on all juices I've used in the Visions.

 

[Revealed]

Actually, I gave it some thought and maybe it is due to the carto getting burned?

 

[440BB]

I'm thinking that it might be leftovers from vaporizing the liquid, possibly some carmelization mixing in with the unvaped liquid. When you overfill a filler based carto and then blow it out, the thicker dark stuff from the bottom always comes out dark for me.